Narrow-line Seyfert 1 (NLS1) galaxies are young active galactic nuclei (AGN). They harbour low-mass black holes accreting close to the Eddington limit and are preferentially hosted by spiral galaxies. So far ~20 NLS1 galaxies have been detected at gamma-rays, confirming the presence of powerful relativistic jets in them. This contradicts the conventional view that only supermassive black holes residing in massive ellipticals are able to launch relatistic jets, and therefore a revision of the evolution and unification schemes of AGN is required.
In this thesis large samples of NLS1 galaxies are examined, complemented by targeted studies of smaller samples. Novel radio, near-infrared, and large-scale environment data were obtained and, together with archival multifrequency data, used for extensive statistical studies such as correlation analyses. Principal component analysis (PCA) emerged as an excellent tool for studying the diverse NLS1 samples.
We monitored large samples of NLS1 galaxies at 37 GHz in Metsähovi Radio Observatory. This is the largest observing programme of NLS1 galaxies at radio frequencies. 19% of sources selected based on their radio properties and 12% of sources selected based on other criteria were detected, including sources previously (mis)classified as radio-silent. As detections at 37~GHz are indicators of radio emission from a jet, this implies that powerful jets in NLS1 galaxies are more frequent than previously assumed. We also found that the jets of flat-spectrum NLS1 galaxies are less powerful than those of blazars, but when scaled by the black hole mass the jet powers become comparable. Additionally, we discovered a new gamma-ray emitting source in the radio-silent NLS1 sample.
We show that the large-scale environment density affects the radio properties of NLS1 galaxies and that jetted NLS1 sources are more frequently found in denser surroundings. However, on average jetted NLS1 galaxies reside in significantly less dense large-scale environments than other jetted AGN, proving that powerful jets can be triggered in diverse environments. In addition a parameter describing the large-scale environment was included in the PCA of AGN for the first time.
Our near-infrared imaging of sources detected at 37 GHz almost tripled the number of jetted NLS1 galaxies with known host morphologies. All observed NLS1 nuclei reside in spiral galaxies. The fraction of mergers in this sample is significantly higher than among non-jetted NLS1 galaxies, suggesting that interaction may play a role in triggering the jet. The heterogeneity of the NLS1 population could be explained by disparate evolutionary stages induced by interaction.